This invention relates to methods and apparatus for recovering minerals and hydrocarbons from subsurface earth formations, and more particularly relates to improved leaching methods and apparatus for recovering solid and semi-solid and viscous liquid earth materials such as kerogen, high viscosity oil, inorganic ores and the like.
The term "leaching" generally means a process wherein a suitable fluid is percolated through an aggregate mixture of solid materials, and whereby the leaching fluid dissolves and carries away certain selected constituents of the mixture. Thus, subsurface salt deposits may be mined by pumping water down through boreholes extending into the strata of interest, and by thereafter recovering the resulting brine solution. Similarly, subsurface ore bodies may be mined by leaching the strata or interest with an appropriate solvent.
In a larger sense, however, the term "leaching" may conveniently be used to characterize any process or technique wherein a fluid is precolated through such an aggregate of solid or semi-solid materials, to liquify selected ones of such materials, and whereby the substances thus treated are then capable of flowing through and separating from the original mixture. For example, it is well known that sulfur can be "leached" from subsurface earth formations lying at depths which preclude commercial recovery by conventional mining operations. Sulfur is almost completely insoluble in water, but its melting point is only slightly above the boiling temperature of water. Thus, water, "super-heated" to a temperature corresponding to the melting temperature of sulfur, is injected into boreholes to liquify the sulfur, and the melted sulfur is then brought to the surface by conventional flow or pumping techniques.
Similarly, steam injection techniques are often used to recover solid and semi-solid petroleum substances from subsurface earth formations. It is well known that oil and gas are conventionally recovered through boreholes drilled into the formations, whereby pressure in the formation causes the oil to percolate through the rock matrix and into the borehole. If the formation contains kerogen or bitumen, however, or even oil having an abnormally high viscosity, the flow rate of such materials is insufficient to permit their recovery in commercial quantities.
Water is not a solvent for petroleum substances, of course, but it is also well known that oil and the like may be heated to reduce its viscosity. Thus, steam, hot water, hot gaseous hydrogen or carbon dioxide, and the like, may be injected into the formation to heat the oil trapped therein, and to reduce its viscosity to a more desirable level. Although some interaction will occur between the hydrogen and the carbon molecules of the oil, the primary function of the fluid injected for this purpose is to heat the oil, and to increase the in situ pressure of fluids contained in the formation, thus increasing the rate of flow of the fluids through the formation, and thus a technique of this description constitutes "leaching" in the larger sense as hereinbefore explained.
The flow rate of viscous liquids through a porous rock formation depends on many factors, of course, as will be apparent from the following well-known relationship: ##EQU1## wherein Q represents flow rate, r is the radial distance from the center of the borehole to the point at which the hydraulic head (h) is measured, b and k represent the thickness and permeability of the formation, .rho. and .mu. represent the density and viscosity of the oil therein, and .differential.h/.differential.r is a function of hydraulic gradient within the formation. For this reason, there are many large oil deposits which are well known to the industry but which have relatively little commercial value because the high viscosity of the oil and the low value of the hydraulic gradient does not permit recovery at a practical rate of flow.
It will be apparent from the foregoing relationship that if the viscosity of the oil could be lowered, or the hydraulic gradient in the formation increased, flow rate through the formation could be significantly increased and substantial additional oil recoveries would be achieved. For this reason, many attempts have been made to find or devise in situ production techniques or systems for reducing the viscosity of oil of this type or for increasing the hydraulic gradient in the formation or both. It is well known that the viscosity of oil is a function of its temperature, and thus most of these attempts have been directed toward heating the oil within the formation employing such methods that also directly or indirectly increase the hydraulic gradient in the formation.
Referring again to leaching techniques for the recovery of non-organic ores and minerals, it should be noted that most such substances of interest are substantially insoluble in water, and thus the term "leaching" clearly encompasses more than the use of a percolating solvent. For example, uranium occurs in the form of mixed oxides UO.sub.3 and UO.sub.2, which are commonly known as uraninite and carnotite, and which are substantially insoluble in water. If UO.sub.2 is converted to the uranyl form (UO.sub.2.sup.++), however, it will combine with chlorine to produce UO.sub.2 Cl.sub.2 which is quite soluble. Accordingly, a uranium-bearing formation may be impregnated with a ferric chloride solution to produce the following in situ reaction: EQU UO.sub.2 +2FeCl.sub.3 .fwdarw.UO.sub.c Cl.sub.2 +2FeCl.sub.2
Some of the uranium atoms at the interface of the ferric chloride solution and the uranium ore will experience an increase in valence to produce the uranyl radical, and it is these radicals which combine with the chlorine ions in the leaching solution to produce uranyl chloride by an oxidation-reduction reaction. Thus, the process is completed by withdrawing the dissolved uranyl chloride from the borehole, and by thereafter reducing the mixture to recover the uranium itself.
It will be apparent that copper and other such metals can be recovered in a similar manner, even though the particular substance is in the form of an insoluble oxide or other compound. The only requirement is that the formation be capable of impregnation by a leaching solution, and thus the ore of interest must be contained in a strata-type formation.
All of these techniques are, of course, subject to many disadvantages. In the case of steam injection to recover high viscosity oil and the like, it should be noted that formation contacted by the steam is only at the interface between the formation and the borehole, and thus production rates are inhibited for this reason. Even more serious, such techniques usually require as many as ten or more injector wells for each twenty-five acres of area, and heat losses by way of the steel well casings are accordingly substantial. In addition, steam injected into the formation from a conventional borehole will often override the oil in the formation and move directly into the producing wells, necessitating the immediate shutdown of such wells.
If the formation is injected with hot gases such as hydrogen, there is a greater tendency for the heated gas to penetrate more easily and deeply into the formation, and also less tendency for materials such as bentonite to expand and clog the pores of the matrix material. However, there is also a greater tendency for the gas to rise to the top of the formation, and to by-pass the oil therein, especially when the formation contains a fissure or other internal discontinuity.
These and other disadvantages of the prior art are overcome with the present invention, however, and novel and improved leaching techniques and apparatus are accordingly provided herein for more effectively and efficiently recovering ores, high viscosity oil, and other similar mineral substances from subsurface earth formations.